Magnetic resonance imagers in use today are large and expensive. The most expensive component of an MR imager is the arrangement of large, solenoidal super conducting magnets. High quality magnets having high magnetic field strength of about 2 Tesla and magnetic field homogeneity of 0.1 to 1.0 parts per million are essential for producing high quality images. Because of the cost of imagers, magnetic resonance imaging is presently only available for critical clinical cases.
In the industry, emphasis has been placed on the development of less expensive imagers containing lower quality magnets to allow wider application of MR imaging. Lower quality magnets generally have low magnetic field strengths of up to about 1.5 Tesla with magnetic field inhomogeneity of 70 to 100 parts per million. Homogeneity of this order presents problems with image artifacts. A need exists for a reliable and safe method of clinical imaging in inhomogeneous magnetic fields.
An imaging system having slight magnetic field inhomogeneity produces images with spatial distortion and signal to noise problems. A distorted image can be corrected by remapping the misregistered spins back into their true positions. The remapping technique is limited to weakly inhomogeneous fields and requires that the field distortions are known. In addition, remapping involves stretching the image, resulting in a non-uniform signal to noise.
As field inhomogeneity increases, more serious imaging problems arise such as complete signal cancellation by incoherent dephasing. As is known in the art, an imaging technique termed spin-echo may be used to overcome dephasing problems. Spin-echo is a pulse sequence comprising a set of RF pulses of defined timing and amplitude, which are usually repeated many times. An MR signal is collected after each repetition. The pulse sequence for spin-echo begins with an initial 90.degree. RF pulse which yields a free induction decay as a function of time. Then at a time TE/2 after the initial pulse, a 180.degree. RF refocusing pulse is applied. After the 180.degree. pulse, a spin-echo signal forms which reaches its maximum amplitude at a time TE after the 90.degree. pulse. A second spin-echo signal can be obtained by applying another 180.degree. refocusing pulse, and further echoes are possible with additional 180.degree. pulses. Using spin-echoes, phase errors from inhomogeneous magnetic fields can be refocused for each data sample.
Some developments have been made for imaging in inhomogeneous fields in which a single data point in K-space is acquired after each refocusing pulse. This method requires that 65,536 refocusing pulses be applied for a 256.times.256 element image. In clinical applications this pulse sequence would push the patient RF power (SAR) to well beyond acceptable guidelines.
Alternatively, more than one line of K-space can be acquired during a pulse sequence. The technique termed "fast spin-echo" applies one refocusing pulse per line of K-space but multiple lines are acquired following an initial 90.degree. primary excitation. If fast spin-echo is used in a highly inhomogeneous magnetic field, suitable data will be acquired at the center of each K-space line where the center of the spin-echo has refocused the dephased spins. However, the K-space data points extending away from the center of the echo will experience increasing amounts of dephasing from the magnetic inhomogeneity. This results in complete incoherent dephasing of the signal at a distance from the center of the echo and causes blurring and artifacts in the reconstructed image.
A method has been developed for MR imaging in inhomogeneous magnetic fields using spin-echo techniques and the acquisition of many partial line segments in K-space per pulse sequence. The method of the present invention allows shorter imaging times and lower SAR than single data point refocussing methods and higher image quality than fast spin-echo.